Our goal is to understand how the intracellular pathogen Legionella pneumophila manipulates host cellular functions to facilitate its multiplication, thus to provide knowledge for designing novel therapeutic strategies for treating infectious diseases. Intracellular growth of L. pneumophila requires the Dot/lcm protein secretion system that translocates bacterial effectors into infected cells. These effectors are believed to modulate host cellular functions for the establishment of a niche that supports bacterial replication. Our specific hypothesis is that during infection, SidF, a substrate of the Dot/lcm system actively prevents infected cells from undergoing apoptosis. This hypothesis is based on four observations: 1) Apoptosis is initiated in permissive macrophages infected by L. pneumophila but the cells are not extensively apoptotic; 2) Macrophages infected by a sidF mutant are more apoptotic than those infected by wild-type bacterium; 3) SidF interacts with two pro-apoptotic proteins, BNIP3 and Bcl-rambo; 4) Cells expressing SidF are resistant to apoptosis induced by these pro-death proteins or chemical agents . Our specific aims are to: 1. Perform a thorough analysis of infection phenotypes of a sidF mutant. We will examine (i) whether the sidF mutant is defective in acquiring endoplasmic reticulum markers, (ii)whether sidF is required for intracellular growth of L. pneumophila in other types of host cells and (iii) apoptotic response of primary macrophages infected by the mutant, (iv)how depletion of Bcl-rambo affects bacterial intracellulargrowth. 2. Analyze the interactions between SidF and the pro-apoptotic protein BNIP3 and how such interactions affect L. pneumophila intracellular growth. We will examine (i) the effect of SidF binding on the pro- apoptotic protein BNI PS-mediated apoptosis, (ii) how BNI PS-mediated apoptosis affects intracellular growth of L. pneumophila and (iii)the mechanisms of how SidF interferes with the activity of BNIP3. 3. Identify L. pneumophila effector(s) that function synergistically with SidF in modulating host functions. We will identify (i) non-SidF effectors that interact with BNIP3, (ii) proteins necessary for intracellular growth in the sidF deletion mutant background and (iii)proteins that specifically interact with SidF to form a multi- component effector complex.